The AP Product 024-1001 Tank Check is a propane tank gas indicator that uses a magnetic sensor to indicate the level of propane left in the tank. Can be installed on vertically and horizontally mounted tanks. The sensor measures the liquid propane using active sonar technology. The sensor communicates via Bluetooth to smartphone or tablet. Magnetic sensors mount to the center/bottom of your LP tank. Use with 20, 30 or 40 lb propane tanks. Can work on larger tanks, but it will not start reading accurately until the level drops. Free Tank Check App can monitor up to 15 different tanks on your RV, BBQ, patio grill, pool heater, home LP tank, etc. App monitors LP level, sensor battery life and signal strength for each sensor added. Sensor battery life is 1 to 1-1/2 years and is easily replaceable. Runs on a disc battery #CR2032. Weight: 1 lb. Includes (1) magnetic sensor, and (3) propane tank feet. Limited 3-Year Manufacturer Warranty.